Different ACK response times in ath?

[Sam Pierson]

Hi all,

Recently I had to do some measurements and came up with some interesting
results with the ath driver.  I would take two machines associated to an AP
and ping the second with the first.  This produces four data frames:
A --> AP :    echo request
AP --> B:   echo request
B --> AP:   echo response
AP -> A:   echo response

Each of these directed data frames results in an ACK packet being
generated by the destination.  I monitor all traffic using a passive
third laptop, which also records elapsed time between frames
(specifically between data frames and ACK packets).

Under channel 11, which has a lot of traffic, I had these results:
(times refer to elapsed time between data and ACK frames)
Using an IBM TP T42:
    D-link DWL-G630:  ~248us
    Netgear WG511T  ~210us
Using IBM TP X23:
   D-link:   ~208us
   WG511T:  ~210us
   internal prism 2.5:   ~256us
Apple Airport:
    ~250us

Why would these values be different?

Additionally, on channel 6 (with little traffic), when using
one of the machines in hostAP mode:

X24:  netgear:  114us
         dlink:        109us
airport:     ~154us


Shouldn't ACK packets be transmitted a constant delay after
directed data frames have been received?  Isn't this the way
the 802.11 standard is defined?  Why is the slower computer
responding faster than the faster computers when the TPs
are running the same drivers (and versions)?


--
Sam